Threaded fasteners such as bolts and studs are widely used to connect, attach and secure machine subassemblies to each other. Bolts can be used alone (e.g., screwed into threaded holes in a structure) or with complementary threaded elements such as nuts, to apply a compressive force between facing surfaces, to resist shear forces acting to slide one element across the other, or to provide (and/or resist) a combination of forces.
Bolts are usually made of a tough material with a high tensile strength, sized well in excess of the design loads expected to be encountered in the particular application, but wear, fatigue, unexpected loading and over-zealous tightening occasionally cause breakage. If both ends of the bolt are accessible, then the broken bolt can be removed and replaced simply, but it often occurs that a portion of a threaded fastener is left threaded inside a blind hole after a break. Sometimes these fragments can be teased out by careful manipulation with a pick, but more commonly, it is necessary to drill into the fastener body and either collapse the outer threaded shell of the fastener or insert a reverse-threaded extraction tool into the hole to assist in removing the fragment.
Because threaded fasteners are usually made of a tough material, it can be difficult to drill extraction holes. When the assembly containing the broken fastener can be moved, improved drilling results are often obtained by securing the assembly to a sturdy machine such as a drill press. However, broken bolts are often encountered in machinery like vehicle engines, where it is impractical to dismantle the assembly so that the bolt can be drilled on a stationary press.
For situations like this, a hand drill may be the next best alternative, but drilling into a tough, small-diameter cylinder (which is often located in an awkward or practically inaccessible spot) without breaking the drill bit or damaging the threading of the bolt hole, can be challenging. A number of universal guides for drilling these sorts of holes are known in the art (see, e.g., U.S. Pat. No. 7,229,237 to Charles Fulgham) but all suffer from drawbacks as discussed below. An improved tool for drilling broken-fastener extraction holes may be of value in this field.